Process for gluing wood in the manufacture of plywood



Patented Mar. 13, 1934 PROCESS FOR, GLUING woon IN THE- MANUFACTURE or PLYWOOD Paul Behnke, Altona-Elbe, Germany, assignor to Gesellschaft fiir Ueberseehandel m. b. H., Hamburg, Germany No Drawing. Application May 12, 1932, Serial No.

. 610,977. In Germany February 16, 1932 1 Claim.

' The invention relates to .a process of gluing wood, more particularly for the manufacture of plywood and the application of veneers.

In the usual manufacture of plywood with liquid casein glue as binding agent, a layer of liquid casein glue is introduced between the pieces of wood to be glued together, whereupon the struc ture obtained, with the glue still wet, is pressed in a press. Theproducts thus obtained are then subjected to a drying operation usually lasting several weeks in order to remove the moisture which has penetrated the wood, owing to the liquid binding agent which normally consists of up to about 80% of water. In order to obviate this expensive and tedious drying, it has already been proposed to employ, instead of liquid casein glue, foil or films of dried casein glue which are placed between the pieces of wood to be glued together, the structures obtained being thereupon subjected to pressing in heated presses. This process, however, has not become established in practice. In the first place, it is difiicult and expensive to manufacture casein glue foil in a useful-form. Secondly, the thin casein glue foil does not fill up the irregularities in the surface of the wood satisfactorily, whereby the gluing is of inferior quality. Thirdly, high pressures and high temperatures, neither of which is good for the wood, arenecessary in order to effectuseful gluing with foil.

It has now been found that both the tedious drying, which is necessary in the ordinary use of liquid casein glue, and the disadvantages accompanying the use of casein glue foil may be completely obviated by applying the casein glue in liquid form to the pieces of wood to be glued, subjecting the pieces of wood thus prepared immediately to artificial and rapid drying in order to remove the water presentin the glue, and thereupon uniting the pieces of wood thus pretreated either together or to untreated pieces of wood 'by .hot pressing. Due to the immediate artificial drying of the wood provided with a layer of liquid casein glue, this new process, which is more particularly adapted for the manufacture of plywood and the application of veneers of rare wood and the like, produces a tenacious coating of dried casein glue which is anchored in the pores of the wood surface, is of high adhesive power and is exceptionally suitable for gluing by hot pressing. This stage of artificial drying, which preferably takes place on endless belts running through suitably heated drying chambers, takes place so rapidly that the liquid of the applied casein glue is unable to penetrate into the wood, so that drying after gluing is no longer required. In the process according to the invention, it is of particular advantage to employ a casein glue which, even after drying on the wood, is still liquefiable by heating to relatively low temperatures. Such a casein glue is obtained very simply by adding hot hide glue or hot bone glue to ordinary casein glue, the mixture being maintained at a suitable temperature during application to the wood. Instead of hot hide glue or bone glue, however, it is possible to add hide glue or bone glue which is liquid when cold, for example, liquid hide glue made with nitric acid, to the cold casein glue so as to obtain a product which possesses the property of still becoming liquid when heated to relatively low temperatures after drying. Similar results are obtained with casein glue to which syrup, sugar solution or the like has been added, or with casein glue in the manufacture of which alkali silicate has been used instead of quicklime. caustic soda or the like, and which by virtue of its manufacture alone has the property of becoming plastic, or mouldable or liquid after drying when heated to relatively low temperatures. If desired, several of the steps described in the foregoing may also be combined together, for example, a casein glue made with alkali silicate may be mixed with some hide glue which is liquid when cold so as to producea casein glue that is particularly suitable for the process according to the invention.

Example 100 parts of casein are mixed with 40 to parts of liquid .or solid water glass and 350 to 500'parts of water. To the mixture obtained in this way are added in thecold, or while hot, 20'to 100 parts of a 10 to 40 per cent. solution of hide glue which, by the addition of acetic acid, formic acid, nitric acid or the like, has been rendered liquid when cold.

The glue thus obtained is applied in a glueapplying machine of the usual construction to both sides of a pine wood middle layer for plywood. Immediately after the application of the glue, the middle layer is passed by means of an endless belt through a drying passage in which a temperature of about 100 C. prevails and-provision has been made for an intense ventilation. After about 5 minutes, the middle layer leaves the drying passage. It has onboth sides a coating of dried casein glue. The increase in weight corresponds substantially to the dried substance of the applied glue and therefore water has not absorbed by the wood.

Veneers 3 millimetres thick are then applied in a heated press to both sides of the middle layer which has been provided in the described manner with a coating of casein glue on both sides, pressing being continued for about 10 minutes at a temperature of 90 to 100 C. and at a pressure of about 7-9 atmospheres. After cooling, the plywood is ready for use. The glued joint obtained is in every respect satisfactory.

In the above example, blood albumin may be used instead of casein.

The process according to the invention is not limited to the use of casein glue with or without added substances. In the process according to the invention, hide or bone glue which has been made liquid in the cold by the addition of nitric been acid or the like may be employed cess instead of casein glue.

I claim as my invention:

A process of gluing together pieces of wood, more particularly for the manufacture of plywood and for veneering, comprising the use of a glue which when dry is liquefiable by heating to a low temperature, applyingsaid glue when in liquid form to a piece of wood, immediately subjecting the piece of wood to artificial and rapid drying to prevent the liquid of the glue from soaking into the wood and to attain a practically liquid-free glue film on the piece of wood, and then hot pressing the piece of wood to the piece of wood to which it is tobe joined to unite the pieces together.

with good suc- PAUL BEHNKE. 

